Two-piece golf ball

Abstract: By controlling center hardness, surface hardness and compression deformation amount of a core, there can be obtained a two-piece golf ball which has a shot feel similar to that of a thread-wound golf ball and which attains high launch angle.

Description: The present invention relates to a two-piece golf ball. More particularly, it relates to a two-piece golf ball which is superior in shot feel and attains high launch angle. A two-piece golf ball has widely been used by many golfers, e.g., amateur golfers and professional golfers, because of long flight distance obtained and excellent flight performances. However, its shot feel is hard in comparison with a conventional thread-wound golf ball and it is thus a desiratum to improve this feature. Heretofore, much effort has been made in order to make the shot of the two-piece golf ball soft, that is, to make the shot feel similar to that of the thread-wound golf ball. Previously, for example, the cover of the golf ball has been softened or the core has been softened. However, it cannot really be said that a shot feel which is similar to that of the thread-wound golf ball has been obtained by means of conventional techniques. Further, there is also the drawback that the two-piece golf ball is hardly stopped by a short iron in comparison with thread-wound golf balls. The main object of the present invention is to provide a two-piece golf ball having excellent control properties, which is easily flown and is easily stopped by a short iron, by making the shot feel of the two-piece golf ball similar to that of thread-wound golf balls and increasing the launch angle. This object as well as other objects and advantages of the present invention will become apparent to those skilled in the art from the following description. EP-A-0545541 discloses a two piece golf ball comprising a core and a cover as described in the preamble of claim 1. In the present invention, characteristics of the two-piece golf ball such as hardness of the core are controlled to make the shot feel similar to that of the thread-wound golf ball and, at the same time, a golf ball which is easily flown is provided. That is, the present invention provides a two-piece golf ball comprising a core and a cover covering the core, wherein the core is made of a rubber composition comprising 100 parts by weight of a base rubber, from 20 to 45 parts by weight of a co-crosslinking agent and from 0.3 to 5.0 parts by weight of an organic peroxide, wherein the hardness of the core is within the range of from 40 to 57 in the vicinity of its center and from 70 to 95 in the vicinity of its surface, the difference between the hardness in the vicinity of the surface and that in the vicinity of the center is within the range of from 28 to 40, the hardness being measured by means of JIS-C type hardness tester, and wherein the compression deformation amount formed between an initial loading of 10 kg and a final loading of 130 kg is from 3.1 to 4.5 mm. The present invention also provides a method of making a two-piece golf ball as herein described comprising: (i) providing a rubber composition comprising 100 parts by weight of a base rubber, from 20 to 45 parts by weight of a co-crosslinking agent and from 0.3 to 5.0 parts by weight of an organic peroxide; (ii) kneading the rubber composition; (iii)forming a core by vulcanization molding of the kneaded rubber composition in a die wherein the vulcanization is conducted at a vulcanization temperature such that a vulcanization peak is obtained within 5 to 20 minutes, and the vulcanization temperature is from 130 to 180 °C; and (iv) covering the core from step (iii) with a cover. The core of the golf ball of the present invention is made of a rubber composition comprising a base rubber, a co-crosslinking agent and an organic peroxide. As the base rubber in the present invention, there can be used a natural rubber and/or synthetic rubber which have hitherto been used for a two-piece core. Particularly, cis-1,4-polybutadiene rubber having at least 40% of a cis-structure is preferred. If necessary, natural rubber, styrene-butadiene rubber, EPDM, etc. , may be suitable formulated in the polybutadiene rubber. The co-crosslinking agent may be specifically limited and examples thereof include metal salts of unsaturated carboxylic acids, having 3 to 8 carbon atoms (e.g. acrylic acid, methacrylic acid, etc). Among them, zinc acrylate is particularly preferred. The amount of the co-crosslinking agent is 20 to 45 parts by weight based on 100 parts by weight of the base rubber. When the amount is smaller than 20 parts by weight, the hardness of the golf ball is insufficient, which results in an excessively heavy and inferior feel. Further, the durability also is inferior. On the other hand, when it exceeds 45 parts by weight, it is too hard, which results in an inferior feel. Examples of the organic peroxide include dicumyl peroxide, di-t-butyl peroxide and the like. Among them, dicumyl peroxide is particularly preferred. The amount is 0.3 to 5.0 parts by weight, preferably 0.5 to 3.0 parts by weight, based on 100 parts by weight of the base rubber. When the amount is smaller than 0.3 parts by weight, the hardness of the golf ball is insufficient, which results in a heavy and inferior feel. On the other hand, when the amount exceeds 5.0 parts by weight, it becomes too hard, which results in an inferior feel. If necessary, additives such as fillers, antioxidants, etc., may be formulated in the rubber composition used for the golf ball of the present invention. Representative examples of the filler include zinc oxide, barium sulfate, etc and the amount thereof varies depending on the specific gravity and size of the cover and core, and is not specifially limited. Normally, it is 10 to 40 parts by weight based on 100 parts by weight of the base rubber. According to the method of the present invention, the core of the two-piece golf ball can be formed by kneading the rubber composition sufficiently, followed by vulcanization molding in a die. The hardness of the core of the present invention (measured by JIS-C type hardness tester) is within a range of 40 to 57 in the vicinity of the center and 70 to 95 in the vicinity of the surface (within 3 mm away from the surface) and, further, the difference in hardness in the vicinity of the surface and that in the vicinity of the center is within a range of 28 to 40. When the hardness of the core in the vicinity of the center is smaller than 40, the durability is inferior, it is too soft and the shot feel becomes heavy, which results in an inferior feel. When it exceeds 57, the shot feel becomes hard, which results in an inferior feel. When the hardness of the core in the vicinity of the surface is smaller than 70, the shot feel becomes heavy, which results in an inferior feel. On the other hand, when it exceeds 95, the shot feel becomes hard. When the difference in hardness in the vicinity of the surface and that in the vicinity of the center exceeds 40, the durability becomes inferior. Further, regarding the core used for the two-piece golf ball of the present invention, it is necessary that the compression deformation amount formed between initial loading (10kg) and final loading (130 kg) is 3.1 to 4.5mm. When the compression deformation amount is smaller than 3.1mm, the shot feel is hard, which results in an inferior feel. On the other hand, when it exceeds 4.5mm, it is too soft, which results in a heavy feel. Further, the golf ball cannot be flown. Persons skilled in the art can be easily instructed to mold a core so as to satisfy the above essential elements by controlling vulcanization conditions. For this purpose, it is conducted by setting a vulcanization temperature so that a vulcanization peak is obtained within 5 to 20 minutes from the vulcanization initiation time, the time when a die is closed being taken as the vulcanization initiation time. The vulcanization reaction is conducted for 20 to 50 minutes from the vulcanization initiation time. The vulcanization temperature is from 130 to 180°C. By controlling the temperature within this range, the above hardness condition and condition of compression deformation can be satisfied. A two-piece golf ball is obtained by covering the core thus obtained with a cover. The cover is formed by covering the core with a cover material obtained by suitably formulating inorganic oxides such as titanium oxide (TiO 2 ), etc., into a resin component which is mainly composed of an ionomer resin, if necessary. When the core is covered with the above cover, an injection molding method is normally used, however, it is not specifically limited. Further, when the resin component of the cover, which is mainly composed of an ionmer resin, is prepared, the ionomer resin may be used alone as the resin component, or resins such as polyethylene, polyamide, etc., or rubbers may be suitably formulated into the ionomer resin as the main component. The thickness of the cover is not specifically limited, but it is normally 1.4 to 2.7 mm. In case of cover molding, a dimple may be formed, if necessary. Further, if necessary, a paint or marking may be applied after cover molding. As described above, according to the present invention, it is possible to make the shot feel of the golf ball softer by increasing the difference between the surface hardness and center hardness. Also, an effect can be obtained such that the launch angle becomes large at the time of hitting and the golf ball is easily flown and, further, the golf ball is easily stopped by a short iron. The following Examples and Comparative Examples further illustrate the present invention in detail but are not to be construed to limit the scope thereof. Examples 1 to 5 and Comparative Examples 1 to 5 The components of the core formulation shown in Tables 1 and 2 were kneaded to prepare a rubber composition, which was subjected to vulcanization molding in a die under the vulcanizing conditions shown in Tables 1 and 2 to give a core, respectively. The hardness was measured at the center of the core, locations which are respectively 5mm, 10mm and 15mm away from the center and the surface of the core, using a JIS-C type hardness tester. Further, the compression strength of the core was also measured. The core thus obtained was coated with an ioner resin of the cover formulation shown in Tables 1 and 2 by an injection molding. The stiffness and the thickness of the cover were shown in Tables 1 and 2. Further, all "parts" in Tables 1 and 2 are by weight. "BR-01" in the core formulation is a butadiene rubber having a cis-1,4-content of (96%) manufactured by Japan Synthetic Rubber Co., Ltd. Further, "Hi-milane 1605" and "Hi-milane 1706" are ionomer resins commercially available from Mitsui Polychemical Co., Ltd. Evaluation of the resulting golf balls was conducted. The hardness (PGA indication) and the durability index as well as the flight distance, the launch angle and the shot feel (evaluation of feelng) obtained by hitting with respective clubs were evaluated. The durability index was measured as follows. That is, a golf ball was hit repeatedly at a head speed of 45 m/second using a swing robot manufactured by True Temper Co. and the number of times until breakage, cracking, etc. is generated was measured and, further, the resulting value was indicated as an index in case of the value of the golf ball of Example 2 being 100. The flight distance and the launch angle were measured by hitting a golf ball using a commercially available swing robot manufactured by True Temper Co. equipped with a No. 1 wood club, No. 5 iron club and No 9 iron club. The feeling was determined by 20 top professional golfers. The evaluation results of the golf balls of the above Exmaples and Comparative Examples are shown in Tables 3 and 4. As is apparent from the above results, all golf balls obtained in the present invention have a soft feel and a feel that the golf ball is easily flown. On the other hand, all golf balls shown in the Comparative Examples have a hard feel and a feel that the golf ball is hardly flown.

Claims: A two-piece golf ball comprising a core and a cover covering the core, wherein the core is made of a rubber composition comprising 100 parts by weight of a base rubber, from 20 to 45 parts by weight of a co-crosslinking agent and from 0.3 to 5.0 parts by weight of an organic peroxide, wherein the hardness of the core is within the range of from 40 to 57 in the vicinity of its center and from 70 to 95 in the vicinity of its surface, the hardness being measured by means of JIS-C type hardness tester, and wherein the compression deformation amount formed between an initial loading of 10 kg and a final loading of 130 kg is from 3.1 to 4.5 mm, characterised in that the difference between the hardness in the vicinity of the surface and that in the vicinity of the center is within the range of from 28 to 40. A two-piece golf ball as claimed in claim 1, wherein the base rubber is a natural rubber and/or synthetic rubber. A two-piece golf ball as claimed in claim 1 or claim 2, wherein the base rubber is cis-1,4-polybutadiene rubber having at least 40% of a cis-structure. A two-piece golf ball as claimed in any one of claims 1 to 3, wherein the co-crosslinking agent comprises metals salts of unsaturated carboxylic acids, preferably monohydric or dihydric metal salts of unsaturated carboxylic acids having from 3 to 8 carbon atoms. A two-piece golf ball as claimed in any one of claims 1 to 4, wherein the organic peroxide comprises dicumyl peroxide, di-t-butyl peroxide or a mixture thereof. A two-piece golf ball as claimed in any one of claims 1 to 5, wherein the rubber composition further comprises fillers and/or antioxidants. A method of making a two-piece golf ball comprising a core and a cover covering the core as claimed in any one of claims 1 to 6, which method comprises: (i) providing a rubber composition comprising 100 parts by weight of a base rubber, from 20 to 45 parts by weight of a co-crosslinking agent and from 0.3 to 5.0 parts by weight of an organic peroxide; (ii) kneading the rubber composition; (iii)forming a core by vulcanization molding of the kneaded rubber composition in a die, wherein the vulcanization is conducted at a vulcanization temperature such that a vulcanization peak is obtained within 5 to 20 minutes from the vulcanization initiation time, the vulcanization reaction is conducted for from 20 to 50 minutes, and the vulcanization temperature is from 130 to 180°C; and (iv) covering the core from step (iii) with a cover.

Inventors: ENDO SEIICHIRO, HORIUCHI KUNIYASU, ENDO, SEIICHIRO, HORIUCHI, KUNIYASU
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES, LTD.
Application year: 1994
Main classification: A63B37
IPC codes: A63B37